1. Field of the Invention
The present invention relates to a method for making shaped structures with internally coated cavities with an inside diameter in the nanometer to micrometer range and shaped structures made thereby.
2. Discussion of the Background
Tubes having an internal diameter of less than 0.1 mm are referred to as hollow fibers, mesotubes, or nanotubes. These hollow fibers with small inside diameters have been used for separation (e.g., medical dialysis), for gas separation, or for osmosis of aqueous systems (e.g., water conditioning). A discussion of the utility of these fibers may be found in Kirk Othmer, Encyclopedia of Chemical Technology, 4th Edition, Vol. 13, pp. 312–313. The fiber material is usually a porous or nonporous polymer, which act as a semipermeable membrane. In addition, the hollow fibers used for separation purposes typically have a surface area of 100 cm2 per cm3 of volume and an inside diameter of 75 μm to 1 mm.
Hollow fibers are also used in microelectronics. For this purpose, superconducting fibers are used, which have a diameter of about 60 μm are made from superconducting materials by filling polymeric hollow fibers with a compound that acquires superconducting properties after thermal decomposition of the polymer (J. C. W. Chien, H. Ringsdorf et al., Adv. Mater., 2 (1990), p. 305).
Extrusion-spinning methods are usually employed to obtain hollow fibers with an inside diameter of approximately 2 μm; several such extrusion-spinning methods are described in Kirk Othmer, Encyclopedia of Chemical Technology, 4th Edition, Vol. 13, pp. 317–322. However, extrusion-spinning methods are limited in that they are unable to provide hollow fibers with an inside diameter of less than 2 μm.
Very thin fibers can be made by electrostatic spinning techniques; however, this technique is limited in that the resultant fibers do not have cavities. In this case, polymer melts or polymer solutions are extruded in an electric field through cannulas at low pressure. The principles of this technique can be found in European Patent 0005035, European Patent 0095940, U.S. Pat. No. 5,024,789 or International Patent WO 91/01695. Although the electrostatic spinning technique can produce massive fibers with diameters of 10 to 3000 nm, hollow fibers cannot be produced with this technique.
Heretofore, hollow fibers with a very small inside diameter have been obtained only through electrochemical synthesis, such as described in L. A. Chemozantonskii (Chem. Phys. Lett. 297, 257 (1998)), by the methods of supramolecular chemistry (S. Demoustier-Champagne et al., Europ. Polymer J. 34, 1767 (1998)), or with self-organizing membranes as templates (E. Evans et al., Science, Vol. 273, 1996, pp. 933–995). Also, carbon hollow fibers based on fullerene chemistry (carbon nanotubes) with single-walled and multi-walled structures of an individually coiled graphite layer (layer of carbon hexagons condensed with one another on all sides) or concentrically disposed graphite cylinders are described in “Fullerenes and related structures,” Editor A. Hirsch, Springer Verlag 1999, pp. 189–234, or N. Grobert, Nachr. Chem. Tech. Lab., 47 (1999), 768–776.
However, these methods are severely limited, as they are only applicable to special materials and cannot be used for making hollow fibers that are mechanically or chemically stable.
For many applications, as in the separation of gases, it is useful to use hollow fibers with small outside and/or inside diameters, made from various materials suitable for the respective application. In particular, it is especially desired that the materials be resistant to thermal, mechanical, and chemical stresses. In certain cases the hollow fiber material must have a porous structure. In addition, they may be selected from electrical conductors or insulators and can contain polymers, inorganic substances or metals. German Patent 1023456.9 provides a suitable hollow fiber with an inside diameter of 10 nm to 50 μm, obtained from technically usable materials such as polymers, inorganic substances or even metals, together with a method for production thereof. The hollow fibers described in German Patent 1023456.9 preferably have inside diameters of 50 nm to 20 μm, more preferably 100 m to 5 μm, even more preferably 100 nm to 2 μm, or 100 run to 1 μm, 500 nm to 1 μm, 10 nm to 1 μm or 100 nm to 500 nm.
The length of the hollow fibers is dictated by the intended use and usually ranges from 50 μm up to several mm or cm. The wall thickness, or in other words the thickness of the outer walls of the hollow fibers, is variable, and usually ranges from 10 to 5000 nm, preferably 10 to 1000 nm, particularly preferably 10 to 250 nm.
In addition to the very small inside diameters, hollow fibers according to German Patent 1023456.9 exhibit a series of characteristics that make them suitable for use in the fields of medicine, electronics, catalysis, chemical analysis, gas separation, osmosis or optics. For example, the outer walls of the hollow fibers can be composed of diverse materials, such as polymers, metals or metal-containing inorganic compounds. The outer walls may be coated with these materials. Accordingly, the hollow fibers may e composed completely of these materials or can be provided with a plurality of layers of the same or different materials. Because of the very small inside diameter, the surface-to-volume ratio of the hollow fibers is very large.
The method for making hollow fibers according to German Patent 1023456.9 can be performed by providing a fiber of a first decomposable material with at least one coating of at least one further material and then decomposing the first material, with the proviso that the resulting hollow fiber has an inside diameter of 10 nm to 50 μm. In an alternative version of the method according to German Patent 1023456.9, a fiber of a first, decomposable material is first coated. This fiber can comprise a material that can be decomposed by thermal, chemical, radiochemical, physical, biological, plasma or ultrasonic means or by extraction with a solvent. Subsequently, the electrostatic spinning technique can be used for making these fibers. Details on the electrostatic spinning technique can be found, for example, in D. H. Reneker and I. Chun, Nanotechn. 7, 216 (1996). A schematic diagram of an electrostatic spinning apparatus is illustrated in FIG. 1.
However, according to the teaching of German Patent 1023456.9, it is necessary to build up the individual layers of the hollow fibers in successive process steps in order to obtain multi-layer hollow fibers. This leads to considerable complexity in both apparatus and process engineering. For many applications, such as in catalysis, it would be desirable to use shaped structures with internally coated cavities, especially internally coated hollow fibers that have small outside and/or inside diameters, that are obtained from materials appropriate for the respective intended use, and that can be made inexpensively by simple techniques.